Ice core shows speedy climate change.

Scientists drilling deep into Greenland's glacial blanket have pulled up evidence from the last ice age showing that the island's climate underwent extreme shifts in just a year or two. This unexpected finding suggests the globe has the potential to warm and cool much faster than ever anticipated.

"What this shows us is that there are big thresholds or instabilities and we don't know what those are yet. So maybe there are some surprises out there," says Kendrick Taylor of the University of Nevada at Reno.

Taylor and his colleagues reported their unexpected findings this week to a standing-room-only crowd at a meeting of the American Geophysical Union in San Francisco.

The scientists are part of an effort called the Greenland Ice Sheet Project 2 (GISP 2), which seeks to remove a 3-kilometer-long cylinder of ice from the thickest part of the ice sheet covering Greenland (SN: 9/14/91, p.168). That huge glacier formed as snow accumulated layer upon layer over the millennia, gradually compacting into ice. By counting the layers backward, the researchers have traced how temperature, snowfall, and other factors changed year by year back into the last ice age, when glaciers covered much of North America, Europe, and Asia.

The ice began melting about 15,000 years ago, signaling the end of that glacial age. But after several thousand years of warming, the climate plunged back into ice-age conditions during a time known as the Younger Dryas period, which lasted between 13,000 years ago and 11,500 years ago.

Studies of less detailed ice cores previously had shown that the cold conditions of the Younger Dryas ended when temperatures in southern Greenland warmed by 7 [degrees] C over a half-century -- a span then considered short (SN: 6/17/89, p.374). But analysis of ice drilled at GISP 2 this summer shows that modern conditions replaced the glacial ones of the Younger Dryas even faster.

Taylor and his colleagues first uncovered the evidence while measuring the electrical conductivity of the ice core -- which reveals the relative amounts of acids and bases in the ice. A drop in conductivity indicates the presence of neutralizing bases, which are carried by windblown dust. During cold periods in the GISP 2 record, the ice conductivity drops, reflecting the dry, windy, and dusty conditions in the northern hemisphere at that time.

The conductivity data reveal that the climate often shifted in a year or two between dusty, glacial conditions and warmer weather. These rapid changes took place at the end of the Younger Dryas and several times between 20,000 and 40,000 years ago.

Another GISP 2 scientist, Richard B. Alley of Pennsylvania State University in University Park, reported that the annual amount of snow accumulation also changed abruptly at these times. As the climate went from cold to warm, the amount of snowfall jumped by as much as 100 percent in just a few years. More snow falls during warmer intervals because the atmosphere holds more water then, explains Alley.

While the findings show that the climate of central Greenland often switched rapidly from glacial to interglacial conditions, they cannot reveal how much of the globe experienced such changes. The GISP 2 scientists think the same shifts affected broad regions, but researchers will have to collect more data at other places to resolve the question.

Climate experts must also strive to explain the causes of such abrupt climate changes. Many suggest there was something about the ice-age Earth that allowed the climate to jump between two different states by redirecting atmospheric and perhaps oceanic circulation patterns. The new finding raises questions about whether global warming from greenhouse gas pollution could soon knock the climate into a new pattern.

"The lesson to me would be that the atmospheric system clearly has inherent instabilities, and it can clearly change in extremely short times. It ought to add just one more note of caution to proceed slowly," says Gifford Miller, a geologist with the University of Colorado at Boulder.

GISP 2 researcher James White of the University of Colorado says, "I used to tell my students climate could change in their lifetime. Well, now I can tell them that it can change in less time than it takes them to graduate."